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BIOLOGY 

LIBRARY 

G 


BWl'WWIVd 

'A  'N  ' 


UNIVERSITY  OF  PENNSYLVANIA 


A  STUDY  OF  THE  KNEE  JERK 


BY 

EDWIN    B,   TWTTMYER 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OP  THE  GRADUATE  SCHOOL  IN 

PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS  FOR 

THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 


PHILADELPHIA 

THE  JOHN    C.   WINSTON    COMPANY 
1902 


UNIVERSITY  OF  PENNSYLVANIA 


A  STUDY  OF  THE  KNEE  JERK 


BY 

EDWIN   B.   TWITMYER 
.11 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL  IN 

PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS  FOR 

THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 


PHILADELPHIA 

THE  JOHN  C.  WINSTON   COMPANY 
1902 


&TB7.2- 


BIOLOGY 

LIBRARY 

G 


CONTENTS 

PAGE 

I.    THE  NORMAL  KNEE  JERK 5 

II.    VARIATIONS    IN    THE    OSCILLATIONS    FOLLOWING    THE 

KNEE  JERK 20 

III.    THE  KNEE  JERK  WITHOUT  BLOWS  ON  THE  TENDONS.  . .  24 


^4 4322 


I. 


The  knee  jerk  is  immediately  caused  by  the  sudden  and  vigorous 
contraction  of  the  quadriceps  muscle  of  the  thigh.  The  quadriceps 
is  the  great  extensor  muscle  of  the  leg,  forming  a  large  fleshy  mass, 
which  covers  the  front  and  sides  of  the  femur.  Sherrington1  has 
demonstrated  by  experimentation  upon  animals  that  two  of  its 
four  divisions,  the  vastus  internus,  covering  the  inner  surface  of 
the  femur,  and  the  crureus,  covering  the  front  of  the  femur,  are 
chiefly  involved  in  the  knee  jerk  mechanism.  These  two  portions 
are  so  intimately  blended  anatomically  as  to  form  a  single  muscle. 
So  considered  it  is  related  by  its  superficial  surface  with  the  psoas 
and  iliacus,  the  rectus,  satorius,  pectineus,  adductors,  and  fascia 
lata ;  by  its  deep  surface  with  the  femur,  sub-crureus,  and  synovial 
membrane  of  the  knee  joint. 

The  tendons  of  the  different  portions  of  the  quadriceps  unite  at 
the  lower  portion  of  the  thigh,  so  as  to  form  a  single  strong  tendon 
which  invests  the  patella  throughout  its  whole  surface  except 
underneath  where  a  free  articular  surface  is  presented,  and  con- 
tinues downward  to  be  inserted  in  the  rough  bony  prominence 
of  the  tibia.  From  the  lower  border  of  the  patella  to  the  insertion 
in  the  tuberosity  of  the  tibia  it  is  known  as  the  ligamentum  patellae. 
The  patella  may  therefore  be  properly  regarded  as  a  sesamoid 
bone,  developed  in  the  tendon  of  the  quadriceps  and  the  ligamen- 
tum patellae.  Immediately  below  the  lower  edge  of  the  patella — 
one-half  inch  to  an  inch — the  tendon  passes  over  a  slightly 
hollowed  space  in  the  parts  beneath. 

The  nervous  structures  involved  in  the  knee  jerk  mechanism 
have  also  been  determined  with  exactness.  Sherrington1  has 
located  the  position  of  both  the  afferent  and  efferent  conduction 
paths.  He  found  the  afferent  path  to  lie  in  the  posterior  root  of 
the  fifth  lumbar  of  the  Rhesus  monkey,  which  corresponds  to  the 
fourth  of  man.  This  posterior  root  receives  afferent  fibers  from  the 
obturator  and  anterior  crural  nerves,  and  from  the  external  and 
internal  popliteal  nerves./  By  the  method  of  severance  and 
other  modes  of  destruction  of  the  conductivity  of  the  nerves 

1  Journal  of  Physiology,  Vol.  13,  p.  666. 

(5) 


entering  the  root,  e.  g.,  cooling  to  the  freezing  point,  etc.,  Sherring- 
ton  found  that  of  the  fibers  entering  the  root  from  these  sources, 
those  on  which  the  jerk  depends  are  not  from  any  except  the 
anterior  crural  nerve,  and  in  the  anterior  crural  nerve  only  those 
fibers  which  issue  from  the  vastus  internus  and  crureus  muscles. 

The  peripheral  terminations  of  the  sensory  conduction  path 
are  therefore  found  in  the  muscles  whose  contraction  causes  the 
movement.  Further  it  is  probable  that  some  of  these  fibers  also 
terminate  in  the  tendon  of  the  muscle.1 

By  the  same  method  the  efferent  conduction  path  was  found 
in  the  anterior  roots  of  the  fifth  and  fourth  lumbar  nerves  of  the 
Rhesus  (fourth  and  third  of  man)  and  was  traceable  along  the 
anterior  crural  nerve  into  those  of  the  muscular  branches  of  that 
trunk  which  supply  the  vastus  internus  and  crureus  divisions  of 
the  quadriceps,  i.  e.,  in  exactly  the  same  muscles  in  which  the 
afferent  condition  path  has  its  peripheral  termination. 

Pathological  evidence,  offered  by  cases  presenting  degeneration 
of  posterior  nerve  roots  and  certain  portions  of  the  posterior 
columns  of  the  cord  in  which  the  knee  jerk  is  observed  to  be 
entirely  absent,  makes  it  clear  that  the  spinal  center  involved  in 
the  knee  jerk  mechanism  is  situated  somewhere  between  the 
second  and  fifth  lumbar  segments  inclusively.  Mills2  agrees  with 
Edinger,  Starr  and  others  in  placing  it  in  the  second  or  third 
lumbar  segments  or  probably  both. 

A  blow  on  the  patella'  tendon  (a)  stimulates  the  peripheral 
sensory  fibers  terminating  freely  in  the  superficial  tissue,  (6) 
stimulates  the  peripheral  sensory  nerve  fibers  terminating  in  the 
tendon,  (c]  depresses  the  tendon  thereby  drawing  it  taut  and 
exerting  through  its  attachments  a  longitudinal  stress  upon  the 
fibers  of  the  quadriceps,  thus  offering  a  stimulus  to  the  peripheral 
sensory  nerve  fibers  terminating  in  the  muscle  itself,  (d)  by 

1  As  to  the  manner  in  which  sensory  nerve  fibers  terminate  in  the  tendons, 
Philipp  Stohr,  in  Lehrbuch  der  Histologie,  1901,  says:   "Die  Sehnenspindeln 
sind  meist  spindelformige  Auftreibungen  von  Sehnenbundeln,  die  von  einer 
gut  entwickelten  bindegewebigen  Hulle  umgeben  werden.      Das  eine  Ende 
der  Spindel  geht  in  Sehnenbundel  uber,  das  andere  setzt  sich  in  Muskelfasern 
fort.     Die  an  die  Mitte  herantretenden  Nervenfasern  theilen  sich  wiederholt, 
verlieren  ihr  Mark  und  gehen  in  ein  reich  entwickeltes  Astwerk  uber  mit  oft 
keulenformig  angeschwollenen  Enden. 

2  Journal  of  Nervous  and  Mental  Disease,  1899,  p.  142. 


depressing  the  tendon  serves  as  a  mechanical  stimulus  to  the 
quadriceps. 

With  the  given  anatomical  structures  comprising  the  knee  jerk 
mechanism  and  the  given  results  of  a  blow  on  the  ligamentum 
patellae,  three  explanations  of  the  physiological  cause  of  the  knee 
jerk  are  possible. 

1.  The  knee  jerk  is  a  reflex.     Stimulation  of  the  sensory  fibers 
terminating  in  the  superficial  tissue  by  contact  or  by  an  electrical 
current  does  not  produce  the  movement,  consequently  if  the  jerk 
is  a  reflex,  the  peripheral  excitation  has  its  origin  in  the  tendon  or 
muscle  (or  both).     In  either  case  the  course  of  the  afferent  impulse 
is  the  same.     ''From  the  tendon  of  the  patella,  the  excitation  is 
carried  to  the  spinal  cord  by  crural  sensory  fibers,  and  enters  by 
the  dorsal  root  into  a  certain  portion  of  the  dorsal  column,  and 
thence  passes  to  the  dorsal  horn;  next  it  takes  its  course  through 
the  intermediate  gray  matter  until  it  reaches  cells  of  the  ventral 
horn;    and  thence  the  motor  excitation  goes  through  the  motor 
roots  and  crural  nerve  to  the  anterior  muscles  of  the  thigh.1" 
Waller2  objects  to  this  theory  on  the  ground  that  the  time  elapsing 
between  the  blow  and  the  response  is  too  short  for  the  trans- 
mission of  an  excitation  over  a  reflex  arc.    By  exact  measurement 
he  found  the  time  between  the  percussion  of  the  tendon  and  the 
contraction  to  be  between  .03  and  .04  second  and  between  the 
direct  percussion  of  the  muscle  and  the  contraction  .03  second. 
Exner's  reflex,  the  winking  of  the  eyelid,  has  a  latent  time  of 
.05  seconds.     Aside  from  this  one  instance  a  knowledge  of  normal 
reflex  times  is  still  wanting. 

2.  The  knee  jerk  is  a  muscular  contraction  due  to  the  direct 
mechanical  stimulation  of  the  muscle.     On  this  theory  the  func- 
tion of  the  nervous  structures  involved,  i.  e.,  an  afferent  and 
efferent  conduction  path  and  a  spinal  segment,  any  interruption 
of  which  results  in  a  total  loss  of  the  jerk,  is  to  maintain  by  means 
of  a  constant  reflex  influence,  the  tonicity  of  the  muscle.     With 
the  tone  present  the  muscle  reacts  directly  to  the  mechanical 
stimulus.     Opposed  to  this  theory  is  the  fact  that  the  knee  jerk 
may  be  present  when  muscle  tone  appears  to  be  wanting  and 
may  be  absent  in  the  case  of  men  who  apparently  have  a  normal 

1  Mills,  Journal  of  Nervous  and  Mental  Disease,  1899,  p.  142. 

2  Journal  of  Physiology,  Vol.  11,  p.  384. 


:.;{  -%i     ;  ,v.  %  ":  8 

amount  of  tone.1  Further  the  theory  does  not  satisfactorily 
explain  the  facts  of  reinforcement,  e.  g.,  the  increase  in  the  extent 
of  the  kick  when  the  hands  are  clinched  just  before  the  tendon  is 
struck.  Lombard2  made  a  study  to  determine  the  changes  in  the 
tone  and  irritability  of  the  quadriceps  and  found  that  neither 
increased  under  reinforcing  conditions. 

3.  The  knee  jerk  is  first  due  to  the  mechanical  stimulation  of 
the  muscle  and  second  to  the  reflex  excitation,  i.  e.,  the  muscle 
contracting  in  response  to  the  mechanical  stimulus  represents  the 
beginning  of  the  kick,  while  the  reinforcement  or  continuance  of 
the  movement  is  the  result  of  the  reflex  impulse.  Lombard3 
reports  the  results  of  one  case  which  showed  an  irregularity  "which 
one  might  expect  if  when  the  original  contraction  of  the  muscle 
had  reached  its  highest  point  or  when  the  muscle  had  even  begun 
to  relax,  a  second  impulse  had  reached  it,  and  caused  it  to  contract 
still  further."  Although  this  result  lends  support  to  the  theory, 
the  non-observance  of  this  irregularity  in  all  knee  jerk  records  is 
not  evidence  to  the  contrary.  The  reflex  influence  may  gradually 
become  operative  quite  before  the  movement  due  to  the  mechanical 
stimulation  had  reached  its  maximum  height,  in  which  case  the 
irregularity  would  not  appear  in  the  record.  Improved  methods 
of  recording  the  excursion  of  the  leg  may  reveal  different  rates  of 
movement  during  different  divisions  of  the  excursion  and  thereby 
furnish  more  conclusive  evidence  on  this  point.  Some  evidence 
for  this  theory  is  presented  in  Part  III  of  this  study. 

The  following  study  comprises  (1)  an  attempt  to  determine  as 
nearly  as  possible  the  extent  of  the  unaugmented  or  normal  knee 
jerk  for  normal  subjects  and  (2)  a  consideration  of  the  modifica- 
tions it  is  observed  to  undergo  during  an  extended  period  of 
experimentation.  When  the  patella  tendons  are  struck  at  exactly 
the  same  place  with  blows  of  constant  force  and  at  regular  inter- 
vals, no  two  of  the  resulting  knee  jerks  are  of  the  same  extent. 
This  variation  is  usually  referred  to  some  accidental  stimulus 
acting  upon  the  subject,  e.  g.,  a  loud  or  distracting  sound  or  to 
an  idea  to  which  the  subject  directs  his  attention,  an  emotional 
state  of  greater  pr  less  intensity,  any  one  of  which  conditions  may 

1  See  Diagram,  p.  17. 

1  Journal  of  Physiology,  Vol.  10,  p.  122. 

4  American  Journal  of  Medical  Science,  Vol.  93,  p.  88. 


produce  a  variation  in  the  extent  of  the  jerk,  or  in  a  general  way  to 
a  change  in  the  activity  of  the  central  nervous  system.1  From 
the  very  nature  of  the  case  such  sources  of  variation  are  not 
wholly  avoidable.  However  secure  the  subject  may  be  kept 
from  accidental  sensory  stimuli,  the  stream  of  consciousness  is 
never  altogether  within  control  of  the  experimenter  and  the 
organic  processes  of  digestion,  circulation,  etc.,  are  constantly 
producing  some  slight  or  more  profound  modification  in  the 
equilibrium  of  the  nervous  system. 

The  results  of  a  study  made  by  Noyes2  on  the  unaugmented 
knee  jerk  in  sleep  in  a  case  of  terminal  dementia  suggest  a  more 
definite  explanation  for  the  variations  occurring  in  kicks  following 
in  close  succession  when  the  conditions  remain  precisely  the  same. 
Noyes  obtained  the  knee  jerk  curve  and  the  Traube-Hering  curve 
for  the  same  period  and  found  a  well-marked  coincidence  between 
them.  A  comparison  shows  that  the  Traube-Hering  curve 
descends  lowest  in  that  part  of  the  group  of  knee  jerks  where  the 
kicks  are  longest  and  at  the  place  where  the  Traube-Hering  curve 
is  highest  the  knee  jerks  are  much  diminished.  A  rise  in  the 
Traube-Hering  curve  indicates  increased  blood  pressure  in  the 
arm,  and  a  fall  in  the  curve  corresponds  to  diminished  blood 
pressure. 

On  the  theory  that  increased  blood  pressure  in  the  extremities 
means  lessened  blood  pressure  in  the  central  nervous  system  there 
is  relative  anaemia  of  the  brain  and  cord  when  the  Traube-Hering 
curve  is  at  its  height,  and  relative  hyperaemia  of  the  brain  and 
cord  when  the  curve  is  lowest.  The  diminished  knee  jerk  would 
then  follow  from  the  lessened  functional  activity  of  the  spinal 
cord  at  the  height  of  the  Traube-Hering  wave,  while  an  increased 
knee  jerk  from  increased  functional  activity  of  the  cord  would 
follow  at  the  low  phase  of  the  peripheral  Traube-Hering  curve. 

Although  these  results  were  obtained  from  a  demented  subject 
there  is  no  reason  for  believing  that  the  same  relation  would  not 

1  In  a  consideration  of  these  variations  Sommer  says:  "Es  mussen  demnach 
in  den  betreffenden  Individuen  Krafte  wirksam  sein,  welche  den  Reflex 
hemmen  oder  verstarken.     Diese  Curvenreihen  sind  der  einfachste  Ausdruck 
eines  variablen  Momentes  im  Nervensystem,  vermoge  dessen  bei  gleichem 
Reiz  verschiedene  Wirkungen  zustande  kommen." — Psychopatholog.   Unter- 
suchungs-Methoden,  1899,  p.  30. 

2  American  Journal  of  Psychology,  Vol.  IV,  No.  3. 


10 

maintain  between  the  two  curves  for  a  normal  subject  -  provided 
all  cerebral  influences  could  be  successfully  removed  or  inhibited. 
Provided  Noyes'  inferences  as  to  the  influences  affecting  the  knee 
jerk  in  the  demented  case  are  legitimate  it  is  not  evident  why  the 
same  inferences  do  not  apply  to  the  normal  subject.  This  fact 
may  therefore  be  looked  upon  as  one  of  the  sources  of  the  variation 
occurring  in  the  extent  of  the  jerks  in  a  given  group. 

In  1887  Lombard1  made  a  study  or  the  character  and  extent  of 
this  so-called  normal  knee  jerk  and  in  addition  to  variations 
occurring  in  successive  kicks,  found  variations  corresponding  to 
the  time  of  day  at  which  the  experiments  were  made.  This  study 
comprised  a  series  of  experiments  made  on  one  subject,  during 
fourteen  consecutive  days  in  which  the  jerk  of  the  right  leg  was 
examined  seven  times  a  day,  twenty-five  experiments  being  made 
at  each  examination.2  The  hours  chosen  were  as  follows,  viz., 
8.15  immediately  upon  arising;  9.15  soon  after  breakfast;  1.15 
just  before  lunch;  2.15  just  after  lunch;  6.15  just  before  dinner; 
8.00  soon  after  dinner,  and  11  just  before  retiring.  The  averages 
of  the  results  for  the  different  periods  were  as  follows:  25,  65,  43, 
47,  30,  40,  27  mm. 

From  these  results  Lombard  concludes  that  there  is  a  diurnal 
variation  of  the  knee  jerk.  "The  variation  corresponds  with  the 
gradual  loss  of  vigor  which  the  body  as  a  whole  suffers  from 
morning  till  bedtime.  This  decline  is  an  interrupted  one  and 
further  corresponds  to  the  temporary  and  partial  recoveries  which 
the  body  undergoes  as  the  result  of  the  fresh  supplies  of  nutriment 
and  rest  which  it  obtains  at  each  meal." 

Lombard  further  observed  that  the  average  knee  jerk  for  any 
given  day  did  not  always  correspond  with  the  average  kick  of  other 
days  in  the  series.  In  a  comparison  of  the  mean  temperature  and 
mean  barometric  records  for  the  same  period  he  found  that  there 

1  American  Journal  of  Psychology,  Vol.  I,  No.  1. 

2  For  the  assumed  normal  condition  Lombard  had  the  subject  lie  upon 
his  left  side  upon  a  comfortable  couch,  the  back  and  head  being  supported. 
The  right  thigh  rested  in  a  splint  of  plaster  of  Paris,  shaped  so  as  to  conform 
to  the  inner  and  posterior  surface,  and  of  such  a  height  as  to  hold  the  knee 
on  a  level  with  the  hip  joint.     The  right  foot  was  supported  at  the  same 
height  by  a  swing  suspended  by  a  cord  from  the  ceiling.     In  this  position  the 
muscles  were  passive,  and  the  whole  body  was  as  far  as  possible  in  a  state 
of  rest. 


11 

was  a  marked  correspondence  in  three  curves.  In  general  as  the 
temperature  rises  the  knee  jerk  becomes  less  and  as  the  tempera- 
ture falls  the  knee  jerk  becomes  larger.  On  the  other  hand  the 
knee  jerk  rises  and  falls  with  the  barometric  curve. 

A  few  preliminary  series  of  experiments  upon  several  subjects 
is  sufficient  to  establish  the  fact  that  the  knee  jerk  is  subject  to 
marked  individual  differences.1  The  question  as  to  what  may 
be  considered  the  normal  jerk  of  the  normal  subject  therefore 
presents  itself.  In  order  to  study  this  problem,  an  extended 
series  of  experiments  was  performed  upon  seventeen  subjects, 
seniors  in  college  and  graduate  students,  all  of  whom  in  a  healthy 
and  normal  condition.  One  subject,  however,  during  the  period 
of  time  over  which  the  experimentation  extended,  developed 
certain  nervous  symptoms  which  will  be  noted  later.  The  experi- 
ments, divided  into  four  groups,  were  made  on  four  different  days, 
a  group  each  day,  usually  with  an  interval  of  one  week,  but  in 
some  few  cases,  at  an  interval  of  two  or  three  weeks.  The  experi- 
ments on  a  given  subject  were  made  at  the  same  hour  each  day  and 
consequently  always  followed  the  same  amount  of  preceding 
college  work.  A  group  consisted  of  either  forty  or  forty-five 
experiments,  which  were  divided  into  series  containing  five 
experiments  each.  The  initial,  middle  and  final  series,  i.  e.,  the 
first,  fifth,  and  eighth  or  ninth,  were  always  made  under  the 
assumed  normal  conditions,  the  remaining  series  of  the  group 
involving  various  augmenting  conditions,  e.  g.,  muscular  contrac- 
tion, mild  electrical  shock  to  various  portions  of  the  body,  pain 
stimulus,  etc. 

In  all  of  the  experiments  the  force  of  the  blows  on  the  tendons 
and  the  intervals  between  the  blows  were  kept  constant.  The 
subjects  were  directed  to  allow  the  body  to  relax  and  to  avoid 
as  far  as  possible,  giving  active  attention  either  to  the  experiment 
or  to  any  ideas  which  might  come  into  mind. 

In  order  to  eliminate  as  far  as  possible  all  variations  due  to  the 
augmenting  effect  of  accidental  stimuli,  the  experiments  were 

1  Sommer  views  the  knee  jerk  phenomenon  as  the  motor  expression  of  a 
definite  disposition  of  the  nervous  system  and  maintains  "dass  aus  dem 
Nachweis  solcher  Schwankungen  von  motorischen  Reactionen  ein  Schluss 
auf  eine  gewisse  generalle  Disposition  der  Nervensubstanz  eines  Individuums 
gemacht  werden  konnte."  x 


12 

performed  in  a  room  from  which  practically  all  noises  from  other 
parts  of  the  building  and  streets  were  excluded.  The  room  is 
partitioned  off  from  a  larger  room  on  the  top  floor  of  a  building 
by  double  walls,  the  space  between  being  filled  with  sawdust. 
The  entrance  is  closed  by  heavy  double  doors;  the  only  other 
means  of  communication  is  a  small  opening  in  the  wall  directly 
behind  the  chair  upon  which  the  subject  is  seated,  which  permits 
the  passage  of  two  fine  linen  threads,  attached  to  the  heels  of  the 
subject  in  the  experimenting  room,  to  the  recording  apparatus  in 
the  room  adjoining. 

The  room  is  illuminated  by  a  single  incandescent  lamp  so  placed 
that  it  does  not  shine  directly  on  the  eyes  of  the  subject.  The 
walls,  ceiling  and  carpet  are  neutral  gray  in  color.  Ample  ventila- 
tion is  provided. 

The  subject  is  comfortably  seated  in  a  Morris  chair,  the  legs  of 
which  had  been  lengthened  about  18  inches.  The  back  of  the 
chair  is  inclined  at  an  angle  of  45  degrees.  In  this  semi-reclining 
position  the  greater  portion  of  the  weight  of  the  body  is  supported 
upon  the  pelvis,  thus  reducing  the  pressure  on  the  flexor  muscles 
of  the  under  part  of  the  thigh  to  a  minimum;  the  knees  extend 
some  distance  beyond  the  edge  of  the  seat;  the  lower  legs  are 
thus  free  to  swing  back  and  forth  without  striking  the  edge  of  the 
seat,  the  feet  clearing  the  floor.  The  head  is  supported  in  a 
comfortable  position  and  the  arms  are  allowed  to  rest  extended 
on  the  broad  flat  arms  of  the  chair.  The  cushion  is  slightly 
hollowed  out  at  the  edge  of  the  seat  to  conform  to  the  contour  of 
the  legs,-  thereby  preventing  them  from  being  moved  out  of 
position.  In  this  position  the  muscles  of  the  body  are  thoroughly 
relaxed  and  the  distracting  effects  of  complicated  harness  and 
apparatus  are  avoided. 

The  bell,  the  use  of  which  is  described  later,  was  fastened  to 
the  wall  just  back  of  the  subject  in  the  median  plane  of  the  head 
and  body  and  on  a  level  with  the  head.  Its  ring  consisted  of  a 
single  tap  of  moderate  intensity. 

The  apparatus  employed  to  give  the  blow  on  the  tendons  con- 
sists of  two  hammers  swinging  as  pendulums  from  a  supporting 
frame.  This  frame  is  constructed  of  two  telescoping  upright  rods, 
secured  in  heavy  iron  bases,  resting  on  the  floor,  and  connected 
above  by  a  horizontal  brass  bar.  The  frame  is  moveable  and 


13 

after  adjustment  can  be  secured  in  position  by  braces  clamped  to 
the  arms  of  the  chair  and  the  upper  ends  of  the  upright  rods. 
The  clamps  on  the  horizontal  bar  furnish  the  point  of  support 
for  the  hammers.  These  clamps  can  be  moved*  to  any  position 
along  the  bar.  The  hammer  is  made  of  lead,  cylindrical  in  form, 
6  cm.  long  and  2.5  cm.  in  diameter;  the  pole  coming  in  contact 
with  the  tendon  is  spherical  in  shape.  It  is  suspended  from  the 
moveable  clamp  by  a  brass  handle  or  arm  made  of  two  pieces 
which  slide  over  each  other  and  are  fastened  together  by  a  thumb 
screw  to  give  any  desired  length.  When  the  hammer  is  adjusted 
for  the  experiment,  the  pole  barely  touches  the  ligamentum 
patellce.  It  is  then  drawn  back  by  the  experimenter  until  it  is 
caught  by  an  electro-magnet,  also  supported  from  the  moveable 
clamp  on  the  horizontal  bar,  and  which  can  be  further  adjusted 
at  any  height  within  90  degrees  from  the  vertical  position.  When 
thus  caught  by  the  magnet  it  is  held  until  the  current  is  broken 
by  the  interrupting  instrument  in  the  recording  room.  This 
arrangement  of  the  hammers  provides  for  exact  adjustment  and 
makes  it  possible  to  keep  the  force  of  the  blow,  once  determined 
upon,  absolutely  constant  for  all  subjects.  In  the  experiments 
herein  reported,  the  length  of  the  arm  of  the  hammer  was  kept  at 
23  cm.,  the  arc  through  which  the  hammers  swung  at  50  degrees. 
By  means  of  an  instrument  (interrupter)  in  the  recording  room, 
the  signal  bell  and  the  hammers  were  operated  automatically  at 
regular  intervals.  This  instrument  consists  of  a  brass  cylinder, 
19  cm.  in  diameter,  which  is  rotated  at  a  constant  rate  about  its 
vertical  axis  by  means  of  a  weight  and  escapement  mechanism. 
The  circumference  of  the  cylinder  is  surrounded  with  a  number 
of  rows  of  small  holes,  placed  at  regular  intervals.  Brass  pegs, 
6  mm.  long  are  inserted  into  these  holes  and  project  5  mm.  beyond 
the  surface  of  the  cylinder.  The  time  interval  required  determines 
in  which  holes  of  a  row  the  pegs  are  to  be  placed.  During  the 
rotation  of  the  cylinder  these  projecting  pegs  come  into  contact 
with  two  contact  pieces  fastened  on  an  upright.  For  the  present 
experiment  the  upper  contact  piece  was  so  adjusted  that  the  upper 
row  of  pegs  in  the  cylinder  in  passing  closed  momentarily  a  circuit 
thereby  ringing  the  signal  bell;  in  the  lower  contact  piece  the 
peg  of  the  lower  row  broke  a  circuit  momentarily,  causing  the 
electro-magnets  to  release  the  hammers.  By  the  adjustment 


14 

of  these  two  contact  pieces,  the  time  elapsing  between  the  ringing 
of  the  bell  and  the  fall  of  the  hammers  can  be  made  any  desired 
amount.  For  the  experiments  herein  reported  the  interval  was 
.5  second. 

The  recording  apparatus  consists  essentially  of  two  pens  to 
each  of  which  are  fastened  the  strings  attached  respectively  to  the 
right  and  left  heels  of  the  subject  in  the  experimenting  room. 
The  pen  carriage  runs  upon  two  steel  wires  drawn  very  taut  arid 
consists  of  two  parts :  an  ink  reservoir  and  a  pen  which  is  supplied 
by  it  with  a  continuous  flow  of  ink.  The  pen  inscribes  the 
excursion  of  the  foot  upon  an  endless  roll  of  paper  which  passes 
over  the  tracing  surface  beneath  the  pen.  The  carriage  is  drawn 
back  after  the  outward  excursion  by  means  of  a  small  weight 
fastened  to  the  rear  end  of  the  carriage  by  a  string  playing  over  a 
small  pulley.  As  the  total  amount  of  the  friction  of  the  pen  on 
the  paper  and  the  carriage  on  the  wires  is  extremely  small  a  weight 
of  17  grams  is  sufficient  for  the  purpose  and  offers  practically  no 
resistance  to  the  outward  kick  of  the  leg.  The  two  rollers  which 
receive  the  paper  are  connected  by  belts  to  a  pulley  which  the 
person  recording  turns  by  hand ;  by  this  means  the  rate  of  move- 
ment of  the  two  rolls  at  any  one  time  is  the  same  and  the  two 
curves  therefore  correspond.  The  recorder  begins  to  turn  the 
pulley  just  before  the  tendons  are  struck  and  continues  turning 
until  the  legs  have  come  to  rest.  The  first  kick  out  of  the  legs  and 
all  the  subsequent  swings  are  therefore  recorded.  (See  Record 
No.  1.)  When  the  pens  are  at  rest,  a  movement  of  the  paper  will 
cause  a  horizontal  line  to  be  inscribed.  This  base  line  is  a  broken 
line;  its  rise  and  fall  indicates  that  the  leg  after  the  kick  does  not 
always  drop  back  into  exactly  the  same  position.  The  extent  of 
any  given  initial  kick  as  well  as  any  subsequent  swing  is  taken  to 
be  the  distance  from  the  extremity  of  the  line  to  the  base  line 
immediately  preceding  and  in  the  tables  given  is  expressed  in 
millimeters. 

In  order  to  determine  the  individual  differences  in  the  normal 
jerk  an  average  of  all  the  normal  series  in  the  four  groups  was 
obtained.  Since  no  two  kicks  are  of  the  same  extent,  even  when 
taken  under  precisely  similar  conditions,  an  average  of  a  deter- 
mined number  of  trials,  must  be  taken  as  the  index  of  the  extent 
of  the  iSck  under  the  given  conditions.  The  mean  variation  is 


15 


frequently  considerable  but  the  averages  obtained  from  several 
groups  under  similar  conditions  usually  correspond  very  closely. 
Although  the  averages  obtained  in  this  manner  may  not  be  looked 
upon  as  absolute  indexes  of  the  extent  of  the  kick,  they,  nevertheless, 
afford  a  satisfactory  means  of  comparison  between  results  obtained 
from  different  individuals  and  under  varied  conditions. 

The  following  table  gives  the  averages  for  the  seventeen  subjects 
for  both  the  right  and  left  legs,  each  average  representing  the 
results  of  60  experiments: 


TABLE   I. 


Subject 

A|B 

C 

D 

E 

F 

G 

H 

I 

J 

K 

L 

M 

N 

0 

P 

Q 

Right  Leg 

0  !  16.4 

20.7 

24.6 

35.2 

52.8 

68.8 

71.9 

73.3 

81.4 

92 

92.6 

98.2 

103.7 

122.9 

135.5 

148.4 

Left  Leg.. 

0     8.9 

20.8 

28 

22.3 

69.9 

64.3 

73.3 

37.9 

76.6 

95.7 

104.4 

98 

109.4 

136.9 

144.6 

165.3 

The  subjects  have  been  arranged  in  order  with  reference  to  the 
increase  in  the  averages  for  the  right  leg.1  It  will  be  noticed  that 
the  increase  in  the  averages  for  the  left  leg  corresponds  very 
closely.2  This  order  is  kept  in  the  tables  following.  The  average 
normal  jerk  for  the  subjects  reported  therefore  varies  from  0 
to  165  mm.  These  marked  individual  differences  are  not  due 
to  any  of  the  sources  of  variation  found  by  Lombard.3  Although 

1  The  averages  given  for  Subject  D  have  little  or  no  value.     In  Group  I 
this  subject  gave  large  kicks  with  regularity,  the  average  for  all  the  normals 
being  92. 1  and  103.3  for  the  right  and  left  legs  respectively.     In  Group  II  the 
extent  of  the  kicks  was  greatly  diminished  and  in  Groups  III  and  IV  no 
response  was  obtained  under  normal  conditions.     For  the  remaining  subjects 
the  averages  fairly  represent  the  actual  results. 

2  During  the  period  covered  by  the  experimentation  Subject  I,  especially 
at  the  time  when  Groups  III  and  IV  were  employed,  was  suffering  from  some 
slight  sensory  disturbances  which  were  restricted  entirely  to  the  left  side 
of  the  body.     These  disturbances  manifested  themselves  in  fugitive  sensations 
of  pressure  along  the  left  side  of  the  lumbar  and  sacral  regions  of  the  cord 
and  in  numbness  of  the  little  finger,  little  toe  and  restricted  areas  on  the 
abdomen.     A  medical  examination  revealed  nothing  further  than  that  the 
subject  was  suffering  from  nerve  exhaustion  due  to  overwork.     This,  fact 
probably  accounts  for  the  marked  discrepancy  between  the  averages tf or  the 
right  and  left  legs. 

3  American  Journal  of  Psychology,  Vol.  I,  No.  1 


16 

the  subjects  were  experimented  upon  at  three  different  periods 
of  the  day,  no  correspondence  is  found  to  exist  between  the 
extent  of  the  kicks  for  the  different  subjects  and  the  time  of 
day  at  which  the  experiments  were  made.  The  experiments  were 
performed  on  Subjects  B,  C,  F,  G,  K  and  M  between  9  and  11  A. 
M.,  the  period  at  which  Lombard  found  the  largest  kicks  occurring 
with  his  subject.  The  averages  for  B  and  C  are  extremely  small 
while  no  subjects  in  this  number  were  among  those  giving  very 
large  kicks.  The  experiments  were  made  on  subjects  A,  E  and  N 
between  11  and  1  o'clock  and  on  Subjects  D,  H,  I,  L,  O,  P  and  Q, 
most  of  whom  were  among  those  giving  the  largest  kicks,  between 
3  and  5  P.  M. 

Further  the  experiments  were  all  made  during  the  months  of 
February,  March  and  the  early  part  of  April,  while  the  building 
was  being  heated,  the  temperature  of  the  room  being  kept  at  about 
70  degrees.  Consequently  these  individual  differences  cannot 
be  ascribed  to  this  cause  of  variation. 

Finally  the  non-correspondence  in  the  extent  of  the  kicks 
obtained  from  subjects  experimented  upon  at  the  same  period 
of  the  same  day  is  sufficient  evidence  that  the  barometric  condi- 
tions are  not  responsible  for  these  differences. 

From  the  results  it  appears  impossible  to  fix  upon  any  given 
amount  as  representing  the  extent  of  the  normal  knee  jerk  of  the 
normal  man,  with  a  blow  of  given  force.  Nine  of  the  seventeen 
subjects  gave  averages  falling  within  wide  limits,  50  to  110  mm., 
and  in  the  absence  of  more  definite  knowledge  we  may  arbitrarily 
designate  an  average  falling  within  these  limits  as  normal.  There 
is  no  reason,  however,  for  looking  upon  averages  falling  without 
these  limits  as  indicative  of  an  abnormal  condition.  There  may 
even  be  a  total  absence  of  the  jerk  in  apparently  sound  subjects 
(Subject  A).  Wide  departures  from  the  limits,  thus  arbitrarily 
determined,  therefore  have  no  significance.  They  may,  however, 
direct  the  study  of  the  cause  of  these  individual  differences  to  the 
cases  in  which  they  are  most  marked. 

A  careful  examination  showed  that  the  anatomical  formation 
of  the  knees  of  the  different  subjects  varied  slightly,  the  hollow 
beneath  the  ligamentum  patellae  being  more  or  less  well  marked. 
This  of  course  allows  a  greater  or  less  displacement  of  the  tendon 
when  struck,  and  in  consequence  the  quadriceps  is  offered  a 


17 


mechanical  stimulus  of  varying  intensity.  No  correspondence 
between  this  fact  and  the  differences  in  the  extent  of  the  kick 
could  be  observed. 

On  the  assumption  that  the  knee  jerk  is  a  muscular  contraction, 
due  to  a  mechanical  stimulus,  ^differences  in  the  extent  of  the 
kick  for  different  subjects,  when  the  intensity  of  the  stimulus  is 
kept  constant,  must  be  referred  to  a  difference  in  the  tonicity  of 
the  quadriceps,  i.  e.,  its  capacity  to  respond  with  a  contraction  of 
given  energy  to  a  stimulus.  This  tone  is  dependent  upon  certain 
relations  of  the  muscle  to  the  central  nervous  system.  It  may 
be  assumed  that  in  the  normal  subject,  the  tone  of  the  various 


1 


BEEF.HILMN-F 

The  dynamometer  test  was  made  only  with  the  ten  subjects  indicated 
The  knee  jerk  curve  (the  heavy  line)  is  constructed  from  Table  I. 

portions  of  the  musculature  of  the  body  is  relative  uniform.  We 
should  then  expect  to  find  a  correspondence  between  the  work 
done  by  any  two  or  more  groups  of  muscles.  A  comparison 
between  the  contraction  of  the  quadriceps  in  the  knee  jerk  and  the 
contraction  of  the  muscles  of  the  hands  and  arms  in  the  dyna- 
mometer test  gives  no  such  correspondence.  For  many  reasons 
the  dynamometer  test  is  unsatisfactory.  The  ability  to  make  a 
high  record  involves  mental  factors  quite  as  much  as  the  mere  tonic 
condition  of  the  muscles.  The  test,  in  this  instance,  however, 
was  unfamiliar  to  the  subjects  experimented  upon  and  the  results 
may  therefore  be  considered  more  of  an  index  of  the  muscular 
than  of  the  mental  conditions  involved. 


18 


In  the  curve  the  heavy  line  represents  the  average  normal  jerk 
of  the  right  leg  for  ten  subjects.  The  broken  line  represents  the 
average  of  five  trials  with  the  dynamometer  with  maximum 
energy. 

The  wide  divergence  of  the  two  curves  suggests  that  the  knee 
jerk  is  more  dependent  upon  differences  in  the  irritability  or  con- 
ductivity, or  both,  of  the  nervous  structures  involved  in  the  knee 
jerk  mechanism  than  upon  differences  in  muscles  tone.  If  this 
inference  is  legitimate  some  evidence  is  offered  for  the  view  that 
the  normal  jerk  is  more  dependent  upon  the  reflex  influences 
initiated  by  the  blow  on  the  tendon  than  upon  the  response  of  the 
quadriceps  muscle  to  a  mechanical  stimulus. 

A  comparison  of  the  results  obtained  in  the  initial,  middle  and 
final  series  of  a  given  group  shows  that  there  is  usually  a  marked 
decline  in  the  extent  of  the  normal  kick  during  the  course  of  the 
group.  In  the  following  table  the  averages  represent  the  results 
obtained  in  the  above-named  series  for  all. four  groups: 


TABLE   II. 


Subject 

A 

B 

C 

D 

E 

F 

G 

H 

I 

J 

K 

L 

M 

N 

0 

P 

Q 

Initial  Series 

0 

17.7 

24.4 

37 

57.4 

71.7 

69.6 

63.5 

91.4 

92 

90.6 

109.1 

103.9 

120.4 

111.5 

163.3 

152.2 

Middle  Series 

0 

18 

21.7 

34 

26.4 

45.9 

75.5 

81.6 

76.6 

74 

97.6 

93.1 

106.8 

99.6 

127 

133.4 

146.7 

Final  Series.. 

0 

14 

13.1 

2.7 

22 

41 

61.4 

70.6 

56.3 

78.4 

98.9 

79.2 

74.6 

88.5 

126 

134.9 

146.3 

The  initial  series  was  always  taken  after  one  or  two  preliminary 
trials  in  the  adjustment  of  the  apparatus;  the  middle  and  final 
series  were  preceded  by  series  in  which  augmenting  stimuli  of  vari- 
ous kinds  were  employed.  For  three  subjects  (H,  K,  O)  the  average 
of  the  final  series  is  slightly  greater  than  the  initial  series.  For 
the  remaining  subjects  (except  A,  who  gave  no  kicks  at  all)  there 
is  a  decrease  which  is  well  marked  in  most  instances.  For  ten 
subjects,  the  value  representing  the  middle  series  falls,  where  we 
should  expect  to  find  it,  between  the  values  representing  the  initial 
and  final  series. 

This  decrease  may  be  due  either  to  fatigue  of  the  muscles  or 
nerves,  or  both,  or  to  an  accommodation  of  the  subject  to  the 
constantly  recurring  stimuli.  Neither  the  nervous  excitation  nor 


19 


the  muscular  contractions  seem  to  have  been  of  sufficient  intensity 
or  duration  to  have  produced  an  appreciable  fatigue  of  the  struc- 
tures involved.  On  the  other  hand,  the  fact  that  a  series  of 
experiments  under  slightly  unusual  conditions,  when  employed 
after  the  completion  of  a  group,  always  resulted  in  kicks  somewhat 
increased  in  extent,  offers  some  evidence  that  the  accommodation 
of  the  subject  to  the  stimulus  and  the  surrounding  conditions  is 
responsible  for  the  decrease.  It  also  indicates  that  the  accommo- 
dation consists  rather  in  a  mental  quiescence  than  in  a  purely 
physiological  modification  of  the  nervous  system. 

The  increase  in  the  extent  of  the  jerk  when  definite  mental  states 
are  employed  as  augmenting  stimuli  and  the  decrease  in  the  kick 
(frequently  approximating  zero)  when  consciousness  is  relatively 
emptied  of  content  led  Witmer1  to  look  upon  the  kick  as  a  function 
of  its  mental  augmentation.  This  view  finds  corroboration  in 


TABLE   III. 


Subject 

A 

B 

C 

D 

E 

F 

G 

H 

I 

J 

K 

L 

M 

N 

0 

P 

Q 

Group  I 

0 
0 

14.4 
25.1 

12.1 

36.4 

92.1 
0 

16.2 
18.7 

26.8 
51.4 

80.2 
60.9 

70.8 
67.9 

72.1 
96.7 

99.5 
93.1 

112.5 
113.6 

105.5 
103.2 

79.5 
111.7 

102.5 
104.9 

83.7 
180.8 

121.4 
173.9 

191.5 
97.8 

Group  IV 

the  fact  that  the  subjects  whose  results  are  herein  reported,  gave 
testimony  that  mental  activity  seemed  to  wane  during  the  experi- 
mentation and  even  frequently  complained  of  drowsiness  toward 
the  end  of  the  group.  The  absence  of  the  reflex  influences 
dependent  upon  vigorous  mental  activity  may  therefore  account." 
for  the  observed  decrease  in  the  final  series. 

Although  the  extent  of  the  kick  tends  to  decrease  during  any 
one  group  of  experiments  there  is  some  evidence  for  believing  that 
the  experimentation  from  week  to  week  tends  to  develop  the 
reaction  in  certain  subjects.  Table  III  gives  the  averages  of 
the  normal  kicks  in  Groups  I  and  IV.  Subjects  G  and  Q  gave 
decreased  kicks  in  Group  IV  while  Subject  D  failed  to  give  any 
normal  kicks  in  Group  IV  although  reacting  when  augmenting 
stimuli  were  employed.  In  Group  IV  no  response  whatever  could 

1  A  paper  before  the  American  Psychological  Association,  1895. 


20 

be  obtained,  even  with  intense  augmenting  stimuli.  Subjects 
B,  C,  F,  I,  M,  O  and  P  gave  marked  increases  in  Group  IV  while 
the  results  for  the  remaining  subjects  differ  but  little  from  the 
results  of  Group  I. 

II. 

In  addition  to  the  differences  in  the  extent  of  the  initial  kick 
out  of  the  legs  following  blows  of  constant  force  on  the  tendons, 
the  subsequent  swings  of  the  legs  before  coming  to  rest  present 
certain  variations.  Record  I1  is  the  curve  of  one  subject  (Sub- 
ject M)  for  the  right  leg  in  a  normal  series  of  Group  I.  The 
tracing  above  the  base  line  represents  the  actual  excursion  of  the 
leg  outward  from  the  position  at  which  it  is  at  rest.2  Experi- 
ments 1,  4  and  5  yield  records  which  correspond  very  closely  both 
in  the  extent  of  the  initial  kick  and  the  number  and  extent  of  the 
subsequent  swings.  In  Experiment  2  the  height  of  the  initial 
kick  is  equal  to  the  initial  kick  of  Experiment  5,  but  the  first 
subsequent  swing  is  considerably  greater  and  an  additional  subse- 
quent swing  appears.  Experiment  3  also  yielded  three  subsequent 
swings,  but  this  may  be  due  to  the  greater  extent  of  the  initial 
kick.  This  record  is  fairly  typical  of  the  tracings  obtained  from 
the  other  subjects  experimented  upon.  When  first  observed  the 
subsequent  swings  appear  as  merely  the  oscillations  of  the  leg 
too  and  fro  as  a  pendulum  but  the  evidence  furnished  by  the 
graphic  record  of  the  differences  between  the  extent  of  the  first 
secondary  swings  following  initial  kicks  of  equal  extent  disproves 
this  view.  That  these  subsequent  swings  are  not  merely  the 
movements  of  the  legs  coming  to  rest  as  a  pendulum  is  further 
established  by  a  comparison  which  Sommer3  made  between  the 
knee  jerk  curve  and  the  curve  obtained  by  raising  the  leg  of  a 
corpse,  immediatelyafter  death,  releasing  it  and  taking  the  record 
of  the  swinging  leg  in  the  same  manner.  The  curve  in  the  latter 
case  with  the  given  height  consisted  of  five  depressions  below  the 
base  line  and  four  elevations,  the  leg  coming  to  rest  as  a  pendulum 
in  exactly  the  same  place  each  trial.  Equal  elevations  of  the  legs 
were  followed  by  the  same  number  of  subsequent  swings.  On 
the  other  hand,  fewer  oscillations  take  place  after  the  first  kick 

1  See  pp.  38-39. 

*  On  account  of  the  limitations    of    the   recording  apparatus   used   the 
tracings  below  the  horizontal  line  have  no  meaning. 
8  Psychopathologischen  Methoden,  p.  28. 


21 

out  in  the  normal  knee  jerk.  The  curve  obtained  by  Sommer, 
when  the  initial  kick  was  about  the  same  height  as  the  elevated 
leg  of  the  corpse,  consists  of  an  elevation,  a  relatively  less  depres- 
sion below  the  initial  level,  of  a  second  elevation  with  return  to 
the  initial  level. 

Record  II  is  the  result  of  a  normal  series  for  the  same  subject 
taken  in  Group  IV.  Eighty-five  or  more  experiments  were  there- 
fore made  between  the  two  records.  Although  the  conditions  were 
exactly  similar  in  the  two  instances,  it  will  be  .observed  that 
together  with  the  increase  in  the  extent  of  the  initial  kick  (see 
p.  19)  there  is  a  marked  difference  in  the  appearance  of  the  subse- 
quent swings.  Both  the  number  and  extent  of  the  swings  is  greatly 
increased.  In  the  last  three  experiments  of  the  series  the  extent 
of  the  first  secondary  swing  and  in  the  third  experiment  the  se  c  - 
ond  secondary  swing  is  greater  than  the  initial  kick. 

Although  the  secondary  swings  in  the  normal  series  of  later 
experiments  become  actually  greater  than  those  in  experiments 
at  the  beginning  of  experimentation,  as  in  the  case  cited, 
for  a  limited  number  of  subjects,  there  is  a  decided  tendency  for 
the  extent  of  the  secondary  swings  gradually  to  increase  for  all 
subjects  as  the  experimentation  proceeds.  Some  subjects,  how- 
ever, show  more  marked  variation  in  the  number  of  subsequent 
swings.  Record  III  (Subject  H),  which  is  the  record  of  one  experi- 
ment in  a  normal  series  in  Group  III,  even  though  the  initial  kick 
is  less  than  those  of  Record  II  (Subject  M),  shows  a  larger  num- 
ber of  subsequent  swings.  The  record  of  this  one  experiment  is 
typical  of  the  records  obtained  in  all  the  experiments  in  the  normal 
series  of  this  group  for  Subject  H.  The  records  of  the  normal 
series  in  Group  I  (Record  IV)  show  two  and  three  less  subsequent 
swings  following  initial  kicks  nearly  equal  in  extent.  The  increase 
in  the  number  of  swings  develops  gradually  throughout  the  inter- 
vening experimentation. 

These  same  variations  are  much  more  apparent  when  some 
augmenting  stimulus  is  employed  to  reinforce  the  normal  jerk. 
Records  V  and  VI  were  taken  when  the  subject  (Subject  M) 
clinched  both  hands  vigorously  just  before  the  blow  on  the  ten- 
dons. Record  V  shows  the  result  of  a  series  in  Group  I,  Record  VI 
the  result  of  a  series  in  Group  III  for  the  same  subject.  The 
variations  observed  in  the  normal  series  are  again  presented. 
The  subsequent  swings  increase  in  number  and  extent  with  the 


22 

increase  in  the  total  number  of  preceding  experiments.  A  much 
greater  number  of  subjects  give  secondary  swings  larger  than  the 
initial  kick  when  some  augmenting  stimulus  is  employed  than  in 
the  normal  series;  a  marked  increase  is  obtained  in  the  remain- 
ing subjects. 

A  comparison  of  Records  VII  and  VIII  (Subject  H)  taken  in 
augmented  series  in  Groups  III  and  I  respectively  shows  not 
only  an  increase  in  the  extent  of  the  subsequent  swings  but  a 
remarkable  increase  in  the  number  of  swings.  This  record  is 
fairly  typical  of  the  records  obtained  for  the  other  experiments 
in  the  series  mentioned. 

Sommer1  concludes  from  the  marked  differences  between  the 
excursion  of  the  leg  of  a  corpse  when  elevated  and  allowed  to 
swing  until  at  rest  and  the  curve  of  the  knee  jerk,  that  in  the 
latter  instance  some  energy  is  somewhere  operative  which  inhibits 
the  previous  mechanical  pendulum  movements  and  that  this 
inhibition  is  conditioned  by  "  Innervationszustande "  mainly 
cerebral  in  origin,  and  not  by  the  mechanism  of  the  knee  joint. 
Many  records  of  the  knee  jerk,  presenting  variations  in  both  the 
form  and  number  of  oscillations  following  the  initial  kicl?  out  of 
the  legs,  for  the  most  part  obtained  from  subjects  presenting 
neuropathological  conditions,  substantiates  the  view  that  the 
character  of  these  oscillations  as  well  as  the  initial  kick  are  an 
expression  of  a  variable  disposition  of  the  nervous  system.  This 
is  certainly  true  as  a  general  statement,  but  on  the  other  hand 
the  results  of  Sherrington,2  which  show  that  at  the  knee  joint 
excitation  of  the  afferent  fibers  coming  from  one  set  of  antagonistic 
muscles  induces  reflex  tonic  contractions  of  the  opposing  set, 
despite  the  fact  that  the  opponent  muscles  are  not  innervated 
from  the  same  spinal  segment,  seem  to  offer  a  more  definite  and 
immediate  explanation  of  the  phenomenon. 

Severance  of  the  great  sciatic  trunk  in  the  lower  animals  pro- 
duces an  increased  briskness  of  the  knee  jerk.  This  Sherrington3 
found  to  depend  solely  upon  the  cutting  of  that  portion  of  the 
trunk  which  is  destined  for  the  hamstring  muscles.  This  sever- 
ance results  in  a  loss  of  the  tone  of  the  hamstring  muscles  in 
consequence  of  which  there  is  a  relaxation  of  the  flexor  tension 

1  Psychopathologischen  Methoden. 

1  Proceedings  of  the  Royal  Society,  Vol.  52,  p.  563. 

3  Loc.  cit. 


.     23 

on  the  leg.  This  allows  the  leg  to  swing  out  without  opposition. 
Certain  experimental  results  obtained  by  Sherrington  supplement 
this  mechanical  explanation  with  a  more  satisfactory  physiological 
explanation.  In  this  study  he  was  able  to  establish  three  important 
facts  concerning  the  contraction  of  the  extensor  muscles  produced 
by  stimulating  the  motor  nerve  to  the  flexors. 

1 .  If  for  the  excitation  of  the  motor  root  to  the  flexors  a  series 
of  induced  currents  are  employed,  succeeding  each  other  at  a  rate 
slow  enough  to  produce  not  perfect  tetanisation,  but  tremulent 
contraction   of   the   muscles,    the   contraction   obtained   in   the 
extensor  muscles  coincidently  was,  nevertheless,  perfectly  steady 
and  tetanic,  although  not  vigorous. 

2.  If  the  flexor  muscles  are  severed  from  connection  with  the 
knee  joint,  so  that  their  contraction  cannot  affect  the  joint,  and 
if  the  knee  jerk  be  elicited  before,  during  and  after  stimulation 
of  the  motor  root  to  the  flexor  muscles,  during  the  excitation, 
when  those  flexor  muscles  were  contracting,  the  knee  jerk,  brisk 
previously  and  brisk  later,  disappeared  or  almost  disappeared. 

3.  If  the  sensory  spinal  roots  belonging  to  the  hamstring  nerve 
are  severed,  the  stimulation  of  a  motor  root  to  the  hamstring 
muscles  is  no  longer  accompanied  by  contraction  of  the  extensor 
muscles  of  the  knee,  even  when  strong  stimulation  is  employed. 

From  these  facts  Sherrington  concludes  that  the  degree  of 
tension  in  one  muscle  of  an  antagonistic  couple  intimately  affects 
the  degree  of  tonicity  in  its  opponent  not  only  mechanically  but 
also  reflexly,  through  afferent  and  efferent  channels  and  the 
spinal  cord. 

It  can  be  maintained,  therefore,  that  a  blow  on  the  patella 
tendon  primarily  initiates  a  nervous  excitation  which  is  reflected 
in  the  second  or  third  lumbar  segment  into  the  efferent  tracts 
to  the  quadriceps  muscle;  as  a  secondary  result  an  excitation 
passes  down  over  the  intervening  segments  of  the  cord  and  is 
reflected  into  the  division  of  the  great  sciatic  nerve  distributed 
to  the  hamstring  muscles.  This  nerve  impulse  momentarily 
increases  the  tone  of  the  hamstring  muscles,  possibly  produces 
mild  contractions,  consequently  not  only  the  initial  kick  but  also 
the  subsequent  swings  of  the  leg  as  well  are  opposed.  In  the 
corpse  these  reflex  influences  are  totally  absent  and  the  leg  is  free 
to  swing  as  a  mechanical  pendulum. 


24 

Upon  this  theory,  how  can  the  increase  in  the  number  and 
extent  of  the  subsequent  swings  following  the  initial  kick,  observed 
to  take  place  as  the  total  number  of  experiments  is  increased  be 
explained?  Let  us  assume  that  the  repeated  transmission  of  reflex 
excitation  along  a  reflex  arc,  including  the  spinal  segment,  an 
afferent  and  efferent  conduction  path  or  from  a  given  segment 
in  the  cord  to  a  segment  somewhat  removed,  develops  a  pathway 
offering  increasingly  less  resistance.  Evidence  bearing  on  this 
point  is  presented  in  Part  I  of  this  monograph.  A  blow  of  con- 
stant force  on  the  tendons  will  then  result  in  a  more  and  more 
intense  stimulation  of  both  the  quadriceps  and  its  antagonist. 
This  increase  will  be  relatively  constant,  consequently  the  extent 
of  the  initial  kick  is  still  checked  by  the  simultaneous  stimula- 
tion of  the  hamstring  muscles.  The  actual  extent  of  the  kick, 
however,  tends  to  increase  somewhat  with  the  continuation  of 
experimentation  (see  p.  19).  With  the  increased  intensity  of  the 
reflex  influence,  the  flexors  are  thrown  into  more  violent  contrac- 
tion, which  contraction  now  becomes  an  adequate  stimulus  to 
the  sensory  fibers  terminating  in  the  flexors  and  as  a  result  there 
is  a  vigorous  excitation  to  the  cord  and  ultimately  to  the  quad- 
riceps. This  excitation  therefore  contributes  its  quota  to  the 
second  outward  excursion  of  the  leg  and  frequently  is  sufficiently 
vigorous  to  produce  a  kick  larger  than  the  initial  kick.  This 
view  is  fully  corroborated  by  Sherrington's1  experiment  in  which 
he  succeeded  in  greatly  augmenting  the  extent  of  the  knee  jerk 
by  merely  compressing  the  hamstring  muscle  when  it  is  com- 
pletely dissected  away  from  its  attachments,  while  the  nerves 
were  still  intact. 

III. 

During  the  adjustment  of  the  apparatus  for  an  earlier  group 
of  experiments  with  one  subject  (Subject  A)2  a  decided  kick  of 
both  legs  was  observed  to  follow  a  tap  of  the  signal  bell  occurring 
without  the  usual  blow  of  the  hammers  on  the  tendons.  It  was 
at  first  believed  that  the  subject  had  merely  voluntarily  kicked 

1  Proceedings  of  the  Royal  Society,  Vol.  52,  p.  563. 

2  Subjects  A,  B,  etc.,  do  not  correspond  with  the  subjects  thus  indicated  in 
Parts  I  and  II.     The  experiments  reported  here  were  made  the  year  previous; 
the  report  includes  the  results  for  all  subjects  upon  whom  the  series  were 
performed. 


25 


out  the  legs,  but  upon  being  questioned,  he  stated  that  although 
quite  conscious  of  the  movement  as  it  was  taking  place,  it  had 
not  been  caused  by  a  volitional  effort,  and  further,  that  the  sub- 
jective feeling  accompanying  the  movement  was  similar  to  the 
feeling  of  the  movement  following  the  blow  on  the  tendons  with 
the  exception  that  he  was  quite  conscious  that  the  tendons  had 
not  been  struck.  Two  alternatives  presented  themselves. 
Either  (1)  the  subject  was  in  error  in  his  introspective  obser- 
vation and  had  voluntarily  moved  his  legs,  or  (2)  the  true  knee 
jerk  (or  a  movement  resembling  it  in  appearance)  had  been  pro- 
duced by  a  stimulus  other  than  the  usual  one. 

In  a  group  of  experiments,  consisting  of  eight  series,  designed 
primarily  to  study  the  effects  of  motor  augmentation,  three  series 
of  five  experiments  each,  were  planned  to  investigate  the  phe- 
nomenon. In  Series  III  the  subject  was  directed  to  clinch  both 
hands  at  the  sound  of  the  bell  and  twice  in  the  series,  the  third 
and  fifth  experiments,  the  hammers  were  dropped  as  usual  but 
were  caught  before  they  had  struck  the  tendons.  Again  in 
Series  VII  the  subject  was  directed  to  clinch  both  hands  and  the 
hammers  were  caught  in  the  second,  fourth  and  fifth  experiments. 
In  Series  VIII,  a  normal  series,  the  bell  sounding  as  before,  the 
hammers  were  caught  in  the  third  experiment. 

These  experiments  were  performed  one  week  subsequent  to  the 
first  observation  of  the  phenomenon.  The  subject  w^as  in  abso- 
lute ignorance  as  to  the  nature  of  the  experiments  in  the  group 
and  had  no  way  of  knowing  when  the  hammers  would  not  be 
allowed  to  strike  the  tendons.  The  following  table  gives  in 
millimeters  the  results  of  the  experiments  when  the  hammers  were 
not  dropped: 

TABLE   IV. 


Right 

Left 

Series  III.... 

Exp.  3 
5 

130 
255 

193 
195 

Series  VII... 

Exp.  2 
4 
5 

244 
145 
183 

291 
204 
240 

Series  VIII.. 

Exp.  3 

105 

168 

26 


It  will  be  observed  that  in  each  of  the  six  trials  in  which  the 
tendons  were  not  struck  the  subject  responded  with  a  decided 
kick.  To  all  appearances  the  movement  had  the  explosive,  jerky 
character  of  the  kick  following  the  actual  blow  on  the  tendons. 
The  subject  again  testified  that  the  movement  had  not  been  pro- 
duced voluntarily. 

The  same  series  of  experiments  was  then  performed  with  five 
other  subjects  who  had  not  been  made  acquainted  with  the  nature 
of  the  series  and  the  results  obtained  from  Subject  A.  The  fol- 
lowing table  gives  the  results  of  these  trials  for  the  six  subjects: 


TABLE   V. 


A 

1 

c 

I 

i 

E 

] 

t 

Series 

Experiment 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

III 

3 

130 

193 

40 

45 

60 

5 

255 

195 

2 

244 

291 

61 

70 

98 

5 

VII 

4 

145 

204 

10 

5 

183 

240 

vm  

3 

105 

168 

Although  results  were  obtained  from  two  additional  subjects 
under  the  conditions  described,  the  results  obtained  from  Sub- 
ject A  were  not  considered  to  have  been  wholly  confirmed. 
Subjects  C,  E  and  F  gave  no  response  when  the  tendons  were  not 
struck.  Subjects  B  and  D  gave  some  kicks  but  not  with  the 
regularity  of  Subject  A.  They  agreed,  however,  with  Subject  A 
in  saying  that  the  kicks  which  occurred  were  wholly  involuntary. 

In  order  more  fully  to  confirm  the  results  obtained  from  Sub- 
ject A,  two  additional  series  were  employed  in  which  the  bell 
sounded  as  usual  and  the  hammers  were  caught  each  time  before 
striking  the  tendons.  In  each  series  the  bell  struck  every  fifteen 
seconds  with  an  interval  of  two  minutes  between  the  two  series. 
For  the  first  series  the  subject  was  given  no  directions  what- 
ever; for  the  second  series  he  was  directed  to  clinch  both  hands 


27 


in  all  experiments  except  the  fourth.  In  the  latter  series  the 
subject,  without  the  knowledge  of  the  experimenter  at  the  time, 
made  an  attempt  to  inhibit  the  kicks.  The  following  was  the 
result  obtained: 


TABLE   VI. 


Series  I 

Series  II 

Right 

Left 

Right 

Left 

140 

180 

197 

260 

101 

133 

128 

148 

160 

220 

25 

41 

32 

50 

11 

25 

136 

171 

106 

100 

With  the  subject  still  remaining  seated  in  position  during  the 
following  five  minutes  there  was  no  further  movement  of  the 
legs  after  the  tapping  of  the  bell  had  ceased. 

One  week  later,  no  experiments  having  been  performed  on  the 
subjects  in  the  meantime,  a  new  group  was  employed,  four  series 
of  which  contained  experiments  in  which  the  hammers  were  not 
permitted  to  strike  the  tendons.  In  Series  III  the  subject  was 
as  before  directed  to  clinch  both  hands  at  the  sound  of  the  bell, 
tendons  not  struck  in  Experiments  3  and  5;  Series  IV,  the  sub- 
ject said  "ah"  when  the  bell  struck,  tendons  not  struck  in 
Experiments  1,  3  and  4;  Series  VI,  Experiment  4,  slight  pin  prick 
on  the  left  thigh;  Experiment  5,  same  on  right  thigh,  tendons  not 
struck  in  either  case;  Series  VIII  the  subject  was  directed  to 
think  vividly  of  clinching  both  hands  without  making  any  actual 
contractions;  tendons  not  struck  in  Experiments  1  and  4.  Table 
VII  gives  the  results  for  the  six  subjects. 

Comparing  the  results  of  this  group  for  all  the  subjects  with 
the  previous  one  it  will  be  observed  that  more  kicks  were  obtained 
in  proportion  to  the  number  of  experiments  made.  Kicks  were 
recorded  for  Subjects  C  and  F,  both  of  whom  had  given  no  result 
in  the  first  group.  Subject  A  consistently  kicked  each  time  and 
Subject  E  alone  failed  to  give  any  response. 

The  week  following  a  final  group,  consisting  of  eight  series  was 
employed,  in  which  five  of  the  series  contained  experiments  with- 
out the  blows  on  the  tendons.  Series  II,  the  subject  was  directed 


28 


to  clinch  both  hands  at  the  tap  of  the  bell,  the  tendons  not  struck 
in  Experiments  3  and  5.  Series  IV  the  subject  received  a  very 
mild  electric  shock  from  electrodes  held  in  the  hands.  An  irjduced 
current,  resulting  from  an  instantaneous  make  and  break  of  the 
primary  current  of  an  induction  coil  was  used  to  produce  the 
shock  which  was  not  sufficiently  strong  to  cause  apparent  con- 
tractions of  the  muscles  of  the  hands  and  forearms.  The  cur- 
rent was  closed  by  the  experimenter  reacting  to  the  sound  of 


TABLE  VII. 


i 

I 

\ 

c 

| 

I 

> 

I 

E 

F 

Series 

Experiment 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

n 

3 

108 

175 

20 

11 

63 

57 

5 

84 

104 

20 

26 

1 

130 

151 

127 

110 

35 

35 

IV  

3 

143 

110 

26 

32 

4 

87 

121 

7 

10 

VI 

4 

89 

302 

264 

330 

22 

0 

65 

5 

171 

169 

232 

178 

41 

VIII  

1 

125 

190 

4 

174 

360 

50 

44 

the  bell  with  a  telegraph  key;  with  probably  large  variations  the 
shock  occurred  about  .125  second  after  the  stroke  of  the  bell. 
The  hammers  did  not  strike  the  tendons  in  Experiments  2  and  5. 
In  Series  VI  the  subject  clinched  both  hands  at  the  sound  of  the 
bell  and  the  shock,  as  in  Series  IV,  was  employed  in  addition. 
The  hammers  did  not  strike  the  tendons  in  Experiments  1,  3,  4 
and  5.  Series  VII  was  a  normal  series,  the  hammers  being  caught 
in  the  second  experiment.  Series  VIII,  same  conditions  as 
Series  IV,  the  tendons  not  struck  in  any  of  the  five  experiments. 
The  following  table  gives  the  results  for  the  six  subjects: 


29 


TABLE   VIII. 


1 

, 

I 

\ 

( 

| 

I 

) 

1 

B 

1 

p 

Series 

Experiment 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

3 

142 

192 

128 

162 

12 

9 

33 

36 

5 

90 

128 

18 

5 

TV 

2 

228 

360 

255 

305 

184 

110 

45 

31 

12 

25 

51 

65 

5 

107 

130 

185 

206 

142 

112 

22 

10 

52 

8 

2 

20 

1 

140 

160 

220 

245 

97 

80 

16 

12 

18 

VT 

3 

160 

215 

328 

360 

23 

15 

55 

30 

4 

156 

184 

240 

295 

60 

49 

72 

80 

5 

145 

152 

280 

310 

58 

27 

53 

30 

vn 

2 

114 

130 

154 

155 

15 

2 

1 

95 

108 

263 

295 

90 

64 

28 

10 

2 

185 

201 

275 

327 

55 

35 

28 

12 

25 

30 

VIII  

3 

4 

135 

110 

156 
242 

247 
265 

276 
360 

52 
30 

15 
15 









75 
35 

55 
45 

5 

185 

216 

255 

234 

62 

30 

46 

18 

In  the  fourteen  experiments  of  this  final  group  in  which  the 
tendons  were  not  struck,  subjects  A  and  F  gave  decided  kicks 
every  time,  with  the  single  exception  of  Subject  F  with  the  left 
leg  in  Experiment  1,  Series  VI.  Subjects  B  and  C  failed  to 
respond  in  only  two  experiments,  while  Subjects  D  and  E  gave 
three  and  four  kicks  respectively.  A  much  greater  proportion 
of  kicks  was  therefore  obtained  in  this  group  than  in  either  of  the 
two  preceding  groups. 

The  results  of  these  experiments  warrant  the  opinion  that  the 
occurrence  of  the  kick  without  the  blow  on  the  tendons  cannot 
be  explained  as  a  mere  accidental  movement  on  the  part  of  the 
subjects.  On  the  contrary  the  phenomenon  occurs  with  sufficient 
frequency  and  regularity  to  demand  an  inquiry  as  to  its  nature. 

Excepting  in  Series  VI  of  the  second  group  and  Series  IV,  VI 
and  VIII  of  the  third  group,  the  tap  of  the  bell  was  clearly  the 


30 

immediate  stimulus  to  the  movement.  When  the  bell  ceased 
striking  no  further  kicks  occurred  without  the  blows  on  the  ten- 
dons. The  clinching  of  the  hands,  which  was  employed  to  pro- 
duce an  augmentation  of  the  normal  jerk,  even  when  performed 
with  maximum  energy  on  the  part  of  the  subject,  elicited  no 
movements  of  the  legs  when  the  bell  was  not  striking.  In  Series  VI 
of  the  second  group  the  painful  prick  on  the  thigh  and  in  Series  IV, 
VI  and  VIII  of  the  third  group,  the  electric  shock  was  employed 
as  the  augmenting  stimulus.  Subsequent  attempts  to  produce  the 
kicks  with  these  stimuli  when  the  bell  was  not  striking  were  some- 
times successful  and  it  may  be  questioned  whether  these  stimuli 
did  not  immediately  cause  the  movement  in  the  experiments 
indicated  rather  than  the  stroke  of  the  bell.  Assuming  this  to 
be  the  case  there  is  still  no  reason  for  believing  that  the  move- 
ment differed  in  character  from  the  movements  following  the 
stroke  of  the  bell  alone.  The  painful  prick  on  the  thigh  was 
not  sufficiently  intense  to  cause  the  subjects  consciously  to  react 
in  an  effort  to  remove,  or  get  away  from,  the  stimulus,  and  as  a 
reflex  movement  in  response  to  a  pain  stimulus,  the  reaction  would 
naturally  have  manifested  itself  in  withdrawal  of  the  thigh  from 
the  stimulus  or  perhaps  a  movement  of  the  entire  body  rather 
than  in  a  simple  kick  out  of  the  legs.  The  induced  current 
employed  was  not  strong  enough  to  be  painful  and,  excepting  a 
few  instances,  produced  no  perceptible  contractions  of  the  muscles 
of  the  hands  and  arms,  much  less  of  the  body  and  lower  extremities. 
The  movement  in  the  first  instance  cannot,  therefore,  be  viewed 
either  as  a  voluntary  effort  or  a  general  reflex  movement  in 
response  to  a  painful  stimulus  nor  in  the  second  instance  as  a 
movement  due  to  the  diffusion  of  the  current  through  the  body 
to  the  muscles  of  the  legs.  We  are  therefore  justified  in  looking 
upon  the  movements  following  these  stimuli  as  identical  in  char- 
acter to  the  movements  obtained  with  the  bell  alone  or  with  the 
bell  and  some  voluntary  muscular  contraction  as  a  source  of 
augmentation . 

Can  these  movements,  then  be  looked  upon  as  the  true  knee 
jerk  phenomenon  in  which  the  appropriate  stimulus,  i.  e.,  a  blow 
on  the  tendons  has  been  replaced  by  a  new  and  unusual  stimulus  ? 

These  movements  are  not  the  result  of  voluntary  effort. 
Each  subject  gave  unqualified  testimony  on  this  point.  This 
testimony  is  further  corroborated  by  the  fact  that  the  subjects 


31 

were  kept  in  absolute  ignorance  as  to  the  nature  of  the  experi- 
ments about  to  be  performed  and  consequently  did  not  know 
when  to  expect  the  blow  and  when  not  to  expect  it.  Further, 
the  position  in  which  the  subjects  were  placed  made  it  impossible 
for  them  to  observe  when  the  hammers  were  about  to  be  caught 
before  striking  the  tendons.  The  kinaesthetic  sensations  result- 
ing from  these  movements  and  the  more  general  subjective  feeling 
could  not  be  distinguished  from  those  aroused  when  the  move- 
ments followed  the  actual. blows  on  the  tendons.  The  subjects 
were,  however,  always  aware  of  the  fact  when  the  tendons  had  not 
been  struck.  Considerable  confidence  may  be  placed  in  this 
testimony,  for  all  the  subjects  had  been  students  of  psychology 
for  two  years  and  were  consequently  familiar  with  the  method  of 
introspective  examination.  Therefore,  as  far  as  the  data  of  intro- 
spection is  concerned,  it  can  be  definitely  stated  that  there  is  no 
difference  between  these  movements  and  the  true  knee  jerk. 

An  effort  on  the  part  of  Subject  A  to  inhibit  the  kicks  in  the 
supplementary  series  following  Group  II  and  in  all  of  the  experi- 
ments of  Group  III  was  wholly  unsuccessful.  (The  attempt  to 
inhibit  the  movement  was  the  subject's  own  initiative  and  was 
not  reported  to  the  experimenter  until  the  experimentation  had 
been  discontinued.)  Later  experiments  on  another  subject  com- 
pletely confirmed  this  result.  This  corresponds  exactly  with 
the  well-known  fact  that  the  knee  jerk  cannot  be  voluntarily 
inhibited  without  actual  contraction  of  the  flexor  muscles  of 
the  thigh. 

To  the  observer  these  movements  resembled  the  knee  jerk 
in  every  detail.  The  characteristic  jerky  or  explosive  appearance' 
was  very  evident.  When  the  tracings  on  the  record  were  com- 
pared no  difference  could  be  discovered.  If  differences  really 
exist  a  much  more  rapid  recording  device  will  be  required  to  detect 
them.  In  proportion  to  the  extent  of  the  kick,  the  number  and 
appearance  of  the  subsequent  swings  of  the  leg  correspond  to 
the  number  and  appearance  of  the  swings  following  the  knee  jerk. 

Except  in  a  few  cases  the  average  extent  of  the  kicks  without 
the  blow  on  the  tendons  was  somewhat  less  than  that  of  the  kicks 
following  the  blow.  The  following  tables  make  a  comparison  of 
the  extent  of  the  kicks  in  the  two  cases.  The  series  in  which 
Subjects  A,  B,  C  and  F  kicked  every  time,  without  a  blow  on  the 
tendons,  were  selected  for  the  tabulation. 


32 

TABLE   IX. 


SUBJECT  A 

Tendons  Not  Struck 

Tendons  Struck 

Primary 

Secondary 

Primary 

Secondary 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Group  I,  Series  iii  and  vii  (subject  clinch 
both  hands)                                              Av.  5 

211.4 

224.6 

225.8 

248 

228.5 

282.7 

277 

302 

Group  II,  Series  ii  (subject  clinch  both  hands)  .  Av.  2 

96 

139.5 

65 

91.5 

164.5 

255 

224 

243 

Group  II,  Series  iv  (subject  said  "ah")  ...       .  Av.  2  ' 

136.5 

130.5 

161.5 

193.5 

99 

134.5 

180 

273 

Group  II,  Series  viii  (thinking  of  clinching 
hands)  Av.  2 

150 

275 

231 

281 

173.5 

296.5 

311.5 

303.5 

Supplementary  (normal)  Av.  5 

113.8 

150.8 

111 

143.4 

118.2 

123.4 

201.8 

274.4 

Group  III,  Series  vi  (clinch  hands  and  electric 
shock)  Av  4 

150 

187.5 

206.5 

285.7 

165 

177 

230 

315 

Group  III,  Series  viii  (electric  shock,  both  hands)  .  . 

142 

184.6 

197.6 

267.2 

154.3 

253 

313 

356 

TABLE   X. 


SUBJECT  B 

Tendons  Not  Struck 

Tendons  Struck 

Primary 

Secondary 

Primary 

Secondary 

Right 

Left 

Right 

Left 

Right 

Left 

Right 

Left 

Group  III,  Series  vi  (clinch  both  hands  and 
electric  shock)  Av.  4 

267 

302.5 

227 

286.5 

287 

312 

357 

360 

Group  III,  Series  viii  (electric  shock,  both 
hands)  Av.  5 

261 

298.4 

204 

250.4 

235.6 

243 

281.7 

289 

SUBJECT  C 

Group  III  Series  vi                                        Av  4 

64.2 

42.7 

19.7 

7.5 

142 

85 

25 

15 

Group  III,  Series  viii  Av.  5 

57.8 

31.8 

16.4 

9 

173.3 

173 

80.3 

81.3 

SUBJECT  F 

Group  III  Series  vi                                        Av.  4 

52 

35 

21.5 

14.5 

55 

130 
125 

12 

15 

Group  III,  Series  viii  Av.  5 

41.9 

31.6 

12.4 

11.6 

91.7 

40 

43 

33 

In  a  study  of  the  tables  it  is  observed  that  the  relation  between 
the  extent  of  the  kicks  of  the  right  and  left  legs  corresponds 
almost  exactly  with  the  results  obtained  when  the  tendons  are 
struck.  Subjects  A  and  B  give  a  uniformly  greater  kick  with 
the  left  leg  than  with  the  right  when  the  tendons  are  struck  both 
in  normal  and  augmented  series.  Subject  C  gives  larger  kicks 
with  the  right  leg.  This  relation  maintains  in  the  kicks  obtained 
without  the  blows  on  the  tendons.  The  results  for  Subject  F 
are  not  as  consistent  as  the  results  for  the  other  subjects;  this 
was,  however,  not  unexpected,  for  in  a  long  series  of  normals  and 
augmented  kicks,  the  subject  frequently  failed  to  give  any  response 
to  a  blow  on  the  tendons,  often  kicked  with  one  leg  only  and  when 
results  were  obtained  for  both  legs,  neither  was  consistently  larger 
than  the  other. 

Further,  the  relation  between  the  extent  of  the  initial  kick  out 
of  the  legs  and  the  first  secondary  swing  remains  constant  for 
each  subject  whether  the  movement  follows  the  blow  on  the  ten- 
dons or  whether  it  follows  the  sound  of  the  bell  alone.  Sub- 
jects A  and  B  give  secondary  kicks  somewhat  larger  than  the 
primary  kicks  and  Subjects  C  and  F  vice  versa. 

The  movement  of  the  legs  following  the  tap  of  the  bell,  with- 
out the  blows  on  the  tendons,  has  the  characteristics  of  a  simple, 
immediate  reaction  to  a  stimulus.  Upon  the  unanimous  testi- 
mony of  the  subjects,  it  was  not  produced  voluntarily,  i.  e.,  there 
was  no  idea  of  the  movement  in  consciousness,  antecedent  to  the 
movement  itself.  It  may,  therefore,  be  held,  tentatively  at  least, 
that  the  movement  is  a  reflex  action.  The  afferent  excitation 
must  therefore  reach  the  cord  at  the  level  of  the  medulla  (or  in 
the  case  of  the  pain  stimulus  of  the  cervical  cord)  and  then  passes 
down  to  the  second  or  third  lumbar  segment  in  which  the  cell 
bodies  of  the  efferent  conduction  path  are  located.  Here  then 
we  have  a  new  and  unusual  reflex  arc.  With  a  stimulus  of 
moderate  intensity,  a  reflex  movement  is  always  restricted  to  the 
group  of  muscles  most  intimately  connected  with  the  sensitive 
part.  The  cell  bodies  of  the  efferent  conduction  path  are  situated 
in  the  same  or  immediately  adjacent  segments  with  the  terminating 
fibers  of  the  afferent  paths.  If  the  intensity  of  the  stimulus  is 
increased,  the  reflex  movement  may  become  more  widely  diffused 
but  is  still  confined  for  some  time  to  muscles  in  the  neighborhood 


34 

of  the  organs  stimulated.  Increase  the  intensity  of  the  stimulus 
still  further  and  the  response  grows  more  and  more  general.  In 
the  case  under  consideration,  however,  the  movements  cannot  be 
viewed  as  a  general  reflex  response  to  an  intensive  stimulus.  In 
no  instance  was  the  sound  of  the  bell  or  the  electrical  shock  suffi- 
ciently intense  to  produce  diffused  movements  even  of  neighbor- 
ing parts;  on  the  contrary,  the  only  movement  of  the  body  was 
confined  to  a  kick  out  of  the  legs  which  always  displayed  a  definite 
character. 

All  the  experiments  which  had  been  performed  in  the  investi- 
gation of  other  questions,  previous  to  each  of  the  three  groups 
reported  in  this  section  of  the  monograph,  may  be  considered  as 
preliminary  or  preparatory.  In  these  experiments  the  sound  of 
the  bell  immediately  preceded  the  blow  on  the  tendons.  One 
hundred  and  twenty-five  such  experiments  had  been  performed  on 
Subject  A  before  the  kick  without  the  blow  on  the  tendons  was 
observed.  After  150  trials  the  phenomenon  was  obtained  from 
Subjects  B  and  D,  after  185  from  E,  after  220  from  C  and  after 
230  from  F.  These  experiments  were  not  in  a  continuous  series 
but  were  performed  at  weekly  intervals  on  five  different  days. 
Later  two  subjects,  not  reported  in  this  study,  were  found  who 
gave  these  kicks  after  thirty  trials  with  the  bell  immediately 
preceding  the  blow.  After  thirty-five  additional  trials  one  of  the 
subjects  kicked  regularly  without  the  blows  on  the  tendons.  The 
occurrence  of  the  phenomenon,  therefore,  depends  upon  the  pre- 
liminary simultaneous  occurrence  of  the  sound  of  the  bell  with  the 
kick  produced  in  the  usual  way,  i.  e.,  a  blow  on  the  tendon. 
After  a  certain  number  of  such  trials,  the  number  varying  for 
different  subjects,  the  association  of  the  sound  of  the  bell  and 
the  kick  becomes  so  fixed  that  the  bell  itself  is  capable  of  serving 
as  a  stimulus  to  the  movement.  Physiologically  the  repeated 
association  of  the  functioning  of  the  motor  cells  in  the  lumbar 
segment  of  the  cord,  -upon  which  the  kick  immediately  depends, 
with  the  excitation  of  centers  in  the  nuclei  of  the  medulla  con- 
nected with  the  auditory  conduction  path,  has  resulted  in  develop- 
ing a  fixed  relationship  between  them.  The  impulse  entering  the 
latter  therefore  finds  an  accustomed  channel  to  the  former. 
"Considerations  of  embryology  and  comparative  anatomy"  in 
the  opinion  of  Dercum,  "point  to  the  conclusion  that  the  nervous 


35 

system,  though  inextricably  complex  and  composed  of  an  almost 
infinite  number  of  parts,  acts  as  a  whole,  and  that  its  parts  are 
so  closely  related  and  interdependent  that  no  one  part  can  move 
unless  every  other  part,  no  matter  how  slightly  or  how  profoundly, 
moves  also."  According  to  this  view  every  incoming  impulse, 
or  whatever  origin,  besides  effecting  a  change  in  the  appropriate 
central  cells,  diffuses  itself  over  the  entire  central  system.  The 
excitation  aroused  by  the  tap  of  the  bell,  therefore,  always  in- 
fluences, to  a  greater  or  less  degree,  the  motor  centers  of  the 
cord.  The  results  of  the  experiments  herein  reported,  however, 
would  seem  to  indicate  that  it  is  only  after  a  habit  of  interaction 
between  the  two  involved  centers  has  been  developed  by  repetition, 
i.  e.,  when  the  connecting  pathway  of  discharge  has  become  well 
worn,  that  the  sound  of  the  bell  alone  is  an  adequate  stimulus  to 
the  movement. 

A  comparison  of  Tables  V,  VII  and  VIII  shows  that  the  number 
of  kicks  obtained  without  the  blows  on  the  tendons  for  each  sub- 
ject gradually  increases  during  the  three  series.  In  the  first  group 
containing  experiments  definitely  planned  to  study  the  phe- 
nomenon, Subject  A  did  not  fail  to  respond  in  any  trial.  An 
equal  number  of  preliminary  experiments  had  been  performed  on 
the  remaining  five  subjects,  but  only  two  gave  kicks  and  these 
with  no  regularity.  For  all  the  subjects,  in  36  possible  times,  11 
kicks,  or  30  per  cent,  were  obtained.  In  Group  II,  after  the  addi- 
tional series  of  Group  I  had  been  performed  under  precisely 
similar  conditions,  the  six  subjects  gave  21  kicks  out  of  a  pos- 
sible 54,  or  40  per  cent.  In  Group  III,  for  which  all  the  pre- 
ceding experiments  can  be  considered  as  preliminary,  the  six 
subjects  gave  kicks  in  87  per  cent  of  the  times  in  which  the' 
tendons  were  not  struck.  Further,  two  of  the  subjects  reacted 
each  trial  and  two  in  all  trials  but  two.  Hence,  not  only  the 
frequency  but  also  the  regularity  of  the  occurrence  increases  with 
the  total  number  of  experiments.  It  may  be  possible  that  the 
employment  of  the  more  exciting  electric  stimulus  in  the  last 
group  partly  accounts  for  the  marked  increase.  It  does  not 
wholly  explain  it  however.  When  this  stimulus  was  tried  on  a 
subject  upon  whom  no  previous  experiments  had  been  performed 
no  responses  were  obtained  without  the  blows  on  the  tendons. 

If  the  knee  jerk  following  a  blow  on  the  tendons  is  a  true  reflex 


36 

it  differs  from  the  movements  of  the  legs  following  the  tap  of  the 
bell  merely  in  that  in  the  latter  an  auditory  excitation  (in  some 
instances  possibly  an  electrical  shock)  has  been  substituted  for 
the  excitation  of  the  peripheral  sensory  fibers  terminating  in  the 
quadriceps  muscle  and  tendon.  There  is  consequently  ah  asso- 
ciation in  a  reflex  arc  of  centers  in  the  medulla  or  cervical  cord 
with  the  motor  cells  of  the  third  lumbar  segment  instead  of  two 
centers  represented  in  the  same  level,  i.  e.,  the  third  lumbar 
segment. 

On  the  other  hand  if  the  knee  jerk  is  not  a  true  reflex  but  merely 
the  result  of  an  immediate  response  to  a  mechanical  stimulation 
of  the  muscle,  and  the  movement  of  the  legs  without  the  blow  is 
a  true  reflex,  a  comparison  of  the  latent  times  of  the  two  move- 
ments should  show  a  marked  difference.  In  the  mere  observation 
of  the  two  phenomena  no  marked  difference  seems  to  exist.  The 
matter  can  be  settled  only  by  an  exact  measurement.  This 
problem  will  be  undertaken  by  the  writer  in  the  near  future. 

The  results  of  the  experiments  herein  reported  seem  to  give 
considerable  support  to  the  theory  held  by  Lombard,  DeWatt- 
ville,  Mills  and  others  that  the  knee  jerk  is  due  first  to  direct 
stimulation  of  the  muscle  and  secondly  to  reflex  influence.  The 
results  show  that  the  extent  of  the  kick  following  a  blow  on  the 
tendons,  with  very  few  exceptions,  is  greater  than  the  kick  fol- 
lowing the  sound  of  the  bell  alone,  all  other  conditions  being  kept 
constant.  If  in  the  knee  jerk,  the  movement  is  initiated  by  the 
contraction  of  the  muscle  due  to  the  mechanical  stimulation  and 
then  reinforced  or  continued  by  a  reflex  excitation  coming  from 
the  spinal  center,  the  difference  between  the  extent  of  the  kicks 
following  the  stroke  of  the  bell  or  the  electrical  shock  and  those 
following  the  blows  on  the  tendons  may  be  taken  to  represent  that 
amount  of  the  movement  which  is  due  to  the  mechanical  stimu- 
lation of  the  muscle.  If  this  is  true  the  extent  of  the  normal 
knee  jerk  depends  largely  upon  the  reflex  influence,  the  mechanical 
stimulation  being  responsible  for  only  the  initial  stage  of  the 
movement. 


LIST  OF  RECORDS 

The  photographs  are  approximately  one- third  size  of  original  records. 

1.  Record  of  five  experiments  under  normal  conditions  in  Group  I  for  Subject 

M,  right  leg. 

2.  Record  of  five  experiments  under  normal  conditions  in  Group  IV  for 

Subject  M,  right  leg.     Eighty-five  experiments  were  performed  between 
Groups  I  and  IV. 

3.  Record  of  one  experiment  under  normal  conditions  in  Group  IV  for  Subject 

H,  right  leg. 

4.  Record  of  three  experiments  for  Subject  H,  right  leg  in  Group  I. 

5.  Record  of  five  experiments  with  motor  augmentation  in  Group  I  for  Sub- 

ject M,  right  leg. 

6.  Record  of  five  experiments  with  motor  augmentation  in   Group  IV  for 

Subject  M,  right  leg. 

7.  Record  of  one  experiment  with  motor  augmentation  in   Group  IV  for 

Subject  H,  right  leg. 

8.  Record  of  one  experiment  with  motor  augmentation  in  Group   IV  for 

Subject  H,  right  leg. 


7 


GENERAL  LIBRARY 
UNIVERSITY  OF  CALIFORNIA— BERKELEY 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

This  book  is  due  on  the  last  date  stamped  below,  or  on  the 

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Renewed  books  are  subject  to  immediate  recall. 


MOV    41955 
OCT251955 


LD  21-100m-l,'54(1887sl6)476 


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